The invention relates to a device for reducing engine torque when shifting a transmission in motor vehicles and, more particularly, to a device for reducing engine torque between the beginning and end of a shift of a transmission in motor vehicles having a control device that continuously detects a beginning of a shift as well as a current engine rpm and calculates in advance an engine rpm anticipated at an end of the shift.
A device of the above-mentioned type is known, for example, from German Patent document DE 28 48 624 C2. In this known device for reducing engine torque, the current engine rpm is detected at the beginning of a shift. The beginning of a shift is recognizable, for example, by a shift command issued by a transmission control device and/or by a change in engine rpm that is characteristic of the beginning of a shift (see for example German Patent document DE 30 18 033 A1). Additionally, in the device known from German Patent document DE 28 48 624 A1, the anticipated engine rpm at the end of the shift, the so-called "synchronous rpm", is calculated in advance. The synchronous rpm can be determined, for example, from the engine rpm detected at the beginning of the shift and from the sudden gear change which forms the basis of this shift. Reduction of engine torque during the shift or between the beginning and end of a shift begins at an initial rpm value and ends at a final rpm value. The beginning and ending rpm values are between the engine torque values detected at the beginning of the shift and the engine torque expected at the end of the shift or calculated in advance, and are also determined as a function of the difference between these engine rpms. The amount of reduction of the engine torque is determined as a function of the value of the engine rpm detected at the beginning of the shift.
In this known device, the engine rpm curve between the engine rpms at the beginning and end of the shift is not taken into account. Since the reduction of engine torque is controlled only by a preset value without taking this engine rpm curve into account, variations in the resultant engine rpm curve from an optimum engine rpm curve are possible, for example, because of mechanical tolerances or because of the temperature dependence of the hydraulic system. Optimum shifting quality or shifting time cannot be guaranteed because of these variations, so that increased wear or adverse effects on comfort occur.
There is therefore needed a device which optimizes the shifting quality and/or the shifting times during a shift with regard to mechanical wear and with regard to temperature requirements.
These needs are met according to the present invention by providing a device for reducing engine torque between the beginning and end of a shift of a transmission in motor vehicles having a control device that continuously detects a beginning of a shift as well as a current engine rpm and calculates in advance an engine rpm anticipated at an end of the shift. The amount of reduction of an engine torque (M.sub.M2) is regulated between the beginning (t.sub.0) and end (t.sub.2) of the shift in such a manner that a predetermined engine rpm curve (v.sub.2) is achieved.
According to the present invention, the amount of reduction of engine torque between the beginning and end of the shift is regulated in such a manner that a preset engine rpm curve is reached. For this purpose, for example, optimum engine torque curves for all possible gear changes are stored in the form of characteristic diagrams from which, depending on the current gear change during a shift, an engine rpm curve is selected and set. The stored engine rpm curves can, for example, contain a dependence upon the difference between the engine rpms present at the beginning and end of the shift or a dependence on the temperature of the hydraulic fluid in the transmission. The preset engine rpm curve can, for example, be in the form of a change in rpm per unit time or in the form of a ratio, for example, a ratio of the difference between the current rpm and the rpm at the end of the shift, to the difference between the rpm present at the beginning and end of the shift.
Reduction of engine torque can be performed in different ways, depending on the value of the amount required to reach the preset engine rpm curve. Preferably, the ignition timing angle is retarded for rapid reduction. It is also possible however, especially with high values for reduction, to reduce the volume of fuel injected and/or the throttle valve opening angle. As a result of the regulation of the reduction of the engine torque according to the present invention, corresponding to an optimum engine rpm curve, clutch wear is reduced and shifting comfort is increased in contrast to the known controls for reducing engine torque.
An advantageous embodiment of the present invention preferably forms the set engine rpm curve from a standard engine rpm curve as a function of the difference between the engine rpm detected at the beginning of the shift and the engine rpm anticipated after the shift.
For example, it is possible to specify only one engine rpm curve for all upshifts and all downshifts, which is changed as a function of the stated difference in engine rpm. For example, the engine rpm curve can be modified in such fashion that when there are large differences, the shifting time is extended and when the differences are small, the shifting time is reduced. With this improvement, according to the invention, by storing only one (or a few) curve or curves instead of plurality of curves, storage space in the control device can be saved.
According to the present invention, the torque of the clutch to be shifted is additionally increased during the shift when the amount of required reduction of engine torque is greater than a maximum possible or maximum admissible amount.
For example, if the motor torque is reduced only by retarding the ignition timing angle, the amount of reduction is limited to a maximum admissible retardation of the ignition timing angle. Therefore, if intervention in the operating parameters of the engine is not sufficient to achieve the desired reduction of the engine torque, the torque in the clutch to be shifted is increased in such a manner that the set engine rpm curve is nevertheless maintained.
The torque in the clutch to be shifted is normally adjusted by pressure control during the shift. According to the invention, for example, the torque in the clutch to be shifted is increased by an offset pressure which is added to the normally set pressure curve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.